Ultra-sensitive detection of contaminants in corrosive gas via intracavity laser spectroscopy (ILS)

ABSTRACT

Contaminants in corrosive gases are detected optically at concentrations below 1 part-per-million (ppm) and extending to a level below 1 part-per-billion (ppb) by using intracavity laser spectroscopy (ILS) techniques. A laser, the ILS laser, is employed as a detector. The ILS laser comprises a gain medium contained in a laser cavity. A gas sample containing gaseous contaminant species is contained within a gas sample cell which is placed inside the laser cavity and on one side of the gain medium. Accordingly, the corrosive gas is prevented from reacting with the components of the ILS laser. The output signal from the ILS laser is detected and analyzed to identify the gaseous species (via its spectral signature). The concentration of the gaseous species can be determined from the spectral signature as well.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of application Ser. No.08/522,963 filed on Sep. 1, 1995 now U.S. Pat. No. 5,689,334.

This application is related to the application Ser. No. 08/675,605 filedon even date herewith. That application concerns an ILS laser pumpedwith a semiconductor diode laser. The present application is directed toa method for detecting the presence of gaseous species in a gas samplecontaining corrosive gas.

TECHNICAL FIELD

This invention relates, generally, to the detection of contaminants ingases, and more particularly, to the high sensitivity detection ofgaseous molecules, atoms, radicals, and/or ions by laser techniquesgenerally termed intracavity laser spectroscopy (ILS).

BACKGROUND OF THE INVENTION

In the preparation of high quality semiconductor material (e.g., siliconfilms) for use in the microelectronics industry, it is well known thatcontaminants must be controlled. Failure to control contaminants, as isalso well known and appreciated, can result in the loss of significantresources as the resultant products are typically not useful for theirintended purposes.

Generally, the starting materials in the fabrication of silicon filmsconsist essentially of gases, typically denoted either "bulk" (e.g.,nitrogen or argon) or "specialty" (e.g., hydrogen chloride, hydrogenbromide, boron trichloride). The successful operation of a fabricationfacility designed to prepare semiconductor materials depends directly onthe purity of the starting gases, as well as the gas handlingcapabilities available to preserve gas purity during the delivery of thegases to the process chamber and while material processing is takingplace. Suitable control of the purity of such starting gases (i.e.,monitoring and inhibiting high levels of contaminants as may becontained in the gases) is essential.

Under many current techniques, such control is achieved after the fact.That is, the silicon films so produced are periodically tested and theproduction line shut down only after such tests reveal the presence ofhigh level contaminants. These processes, as will be appreciated bythose skilled in the art, can lead to the waste of not only startingmaterials but also product which is produced prior to cessation ofproduction. It is therefore desirable to monitor and control thecontaminants as may be contained in such starting gases duringproduction so as unacceptable contaminant levels are observed,production can be immediately, or at least shortly thereafter, halted.

Many molecular, atomic, radical, and ionic species are present in thebulk and specialty gases used in the preparation (e.g., chemical vapordeposition or "CVD") and processing (doping and etching) ofsemiconductor materials that can be viewed as "contaminants." Suchcontaminants can degrade either the quality of the fabricatedsemiconductor material or the efficiency with which the semiconductormaterial is prepared. These contaminant species can interfere with thechemical process directly or even cause particles to be formed in thegas delivery lines or process chamber which subsequently deposit on thesurface of the wafer material causing indirect performance defects.

The first step in controlling and/or eliminating these contaminants istheir detection in the bulk and specialty gases used as startingmaterials. While this is generally recognized, heretofore practicedmethods are generally inadequate. This is due, in large part, to thesituation created by seemingly ever increasing competitive industrystandards which have developed. Specifically, as the size ofmicroelectronic devices has decreased while performance specificationshave been intensified, the requirements for gas purity (i.e., absence ofmicrocontamination) has increased.

Against this backdrop, it will likely be clear that several measurementcriteria are important to detector effectiveness: (1) absolute detectionsensitivity usually stated as parts-per-total number of gas molecules inthe sample (e.g., parts-per-million or number of contaminant moleculesper 10⁺⁶ background molecules); (2) species selectivity or thecapability to measure the concentration of one species in the presenceof other species; (3) rapidity of measurements to obtain a desiredsignal to noise ratio; (4) capability of monitoring contaminants in bothnon-reactive and reactive gases; and (5) linearity and dynamic range ofgas concentrations that can be measured.

The current state-of-the-art devices for contaminant detection (e.g.,water) encompass a variety of measurement techniques. For example,current state-of-the-art devices for water vapor detection utilizeconductivity and electrochemical, direct absorption spectroscopy, andatmospheric pressure ionization mass spectroscopy (APIMS) measurementtechniques. As discussed below, each of these methods fails toadequately address these requirements.

Conductivity and Electrochemical

Conductivity and electrochemical methods by solid-state devices existwhich can detect water vapor at the 1 to 100 ppm range. Conductivity andelectrochemical methods generally require direct physical contactbetween the sample and the device; thus, detection occurs after watermolecules deposit on the solid-state surface. As a consequence, thesedevices do not perform well, if at all, with utilization of reactive orcorrosive gases. In such corrosive environments, the performancelifetimes of these devices are limited since the materials from whichthey are fabricated deteriorate in the presence of the corrosive samplebeing measured. The need to have direct contact with the sample is amajor limitation since it is unlikely that a particular material canperform well and resist deterioration over extended use. Indeed, eventheir performance in non-reactive gases changes and/or deterioratesafter even short exposures to reactive or corrosive gases. The linearityand dynamic range of response are usually limited to about one decade.The detection selectivity of these devices with respect to differentgaseous species also is generally poor since the devices themselves willrespond to a wide range of species without discrimination. Additionally,selectivity is incorporated into the measurements only through whateverchemical selectivity, if any, is embodied in the coatings used to coverthese devices.

Direct Absorption

Direct absorption spectroscopy generally relates to the passing of lightthrough the sample from an external source and measuring the reductionin light intensity caused by molecular, atomic, radical, and/or ionicabsorption in the sample. Detection sensitivity depends directly on thesubtraction of two large numbers (light intensity from the externalsource before it passes through the sample and its intensity after itexits the sample). This limits the detection sensitivity to the extentthat direct absorption is generally considered a low sensitivitymethodology.

APIMS

APIMS, initially used in the analysis of impurities in bulk nitrogen andargon and ambient air for air pollution studies, is now currently usedby semiconductor manufacturers to detect trace levels of moisture andoxygen in inert bulk gases. With APIMS, the sampled gas is bombardedwith electrons, or may be flame and photon excited, to produce a varietyof ions that are then detected directly. Particularly, ionization occursat atmospheric pressure in the presence of a reagent gas in theionization source. APIMS typically exhibits detection sensitivities inthe range of about 10 parts per trillion (ppt) in non-reactive gases.APIMS cannot even be used with reactive or corrosive gas mixtures.Additional disadvantages of APIMS include an average cost between about$150,000 to $250,000, extensive purging and calibration procedures, andthe need for a knowledgeable operator.

Intracavity Laser Spectroscopy

In the context of the present invention, laser technology, specificallyintracavity laser spectroscopy (ILS), is disclosed as being used as adetector (sensor) to detect gaseous species (contaminants) at very highsensitivity levels. While the methods and apparatus disclosed herein areparticularly suited for application in fabrication of semiconductorcomponents, it should be appreciated that the present invention in itsbroadest form is not so limited. Nevertheless, for convenience ofreference and description of preferred exemplary embodiments, thisapplication will be used as a benchmark. In connection with thisapplication, laser technology offers distinct advantages to gaseousspecies (contaminant) detection over known methods and, particularly, towater vapor detection.

In conventional applications of lasers to the detection of gaseousspecies (contaminants), laser produced radiation is used to excite thegas sample external to the laser in order to produce a secondary signal(e.g., ionization or fluorescence). Alternatively, the intensity of thelaser after it passes through a gas sample, normalized to its initialintensity, can be measured (i.e., absorption).

Some twenty years ago, another detection methodology, intracavity laserspectroscopy, was first explored in which the laser itself is used as adetector; see, e.g., G. Atkinson, A. Laufer, M. Kurylo, "Detection ofFree Radicals by an Intracavity Dye Laser Technique," 59 Journal OfChemical Physics, Jul. 1, 1973.

Intracavity laser spectroscopy (ILS) combines the advantages ofconventional absorption spectroscopy with the high detection sensitivitynormally associated with other laser techniques such as laser-inducedfluorescence (LIF) and multiphoton ionization (MPI) spectroscopy. ILS isbased on the intracavity losses associated with absorption in gaseousspecies (e.g., atoms, molecules, radicals, or ions) found within theoptical resonator cavity of a multimode, homogeneously broadened laser.These intracavity absorption losses compete via the normal mode dynamicsof a multimode laser with the gain generated in the laser medium (i.e.,gain medium). Traditionally, ILS research has been dominated by the useof dye lasers because their multimode properties fulfill the conditionsrequired for effective mode competition and their wide tunabilityprovides spectral access to many different gaseous species. Some ILSexperiments have been performed with multimode, tunable solid-statelaser media such as color centers and Ti:Sapphire; see, e.g., D.Gilmore, P. Cvijin, G. Atkinson, "Intracavity Absorption SpectroscopyWith a Titanium: Sapphire Laser," Optics Communications 77 (1990)385-89.

ILS has also been successfully used to detect both stable and transientspecies under experimental conditions where the need for high detectionsensitivity had previously excluded absorption spectroscopy as a methodof choice. For example, ILS has been utilized to examine gaseous samplesin environments such as cryogenically cooled chambers, plasmadischarges, photolytic and pyrolytic decompositions, and supersonic jetexpansions. ILS has been further used to obtain quantitative absorptioninformation (e.g., line strengths and collisional broadeningcoefficients) through the analysis of absorption lineshapes. Some ofthese are described in G. Atkinson, "Intracavity Laser Spectroscopy,"SPIE Conf., Soc. Opt. Eng. 1637 (1992).

Prior art methods of performing ILS, however, while suitable for use inlaboratory settings, are unacceptable for commercial settings. Theconstraints of commercial reality, as briefly noted above, essentiallydictate that such a detector be conveniently sized, relativelyinexpensive, and reliable. Laboratory models fail to fully meet theserequirements.

Additionally, in the commercial setting for which ILS detection can beapplied, the gas sample may be contained in an environment which is notideal. In particular, for industrial processes, the gas sample iscommonly contained in corrosive gas. For example, in the manufacture ofsemiconductor components, the ability to detect the presence of acontaminant (such as water vapor) in the presence of a corrosive gas(such as hydrogen chloride) would be particularly useful.

A laboratory demonstration of the feasibility of using ILS techniquesfor detecting small quantities of water vapor in a nitrogen atmospherewith a Cr⁴⁺ :YAG laser is described in D. Gilmore, P. Cvijin, G.Atkinson, "Intracavity Laser Spectroscopy in the 1.38-1.55 μm SpectralRegion Using a Multimode Cr⁴⁺ :YAG Laser," Optics Communications 103(1993) 370-74. The experimental apparatus utilized was satisfactory fordemonstration of operational characteristics, but undesirable forimplementation in a commercial application as contemplated by thepresent invention, in particular, for the detection of water vapor in acorrosive gas such as hydrogen chloride (HCl).

In accordance with various aspects of the present invention, the presentinvention provides a user friendly, i.e., comparatively simple,detection system, having the advantages of direct absorption techniquesbut with dramatically increased detection sensitivities, capable ofdetecting gaseous species in corrosive samples at a commercially viablecost. In this regard, the present invention addresses the long felt needfor a method and apparatus for the high sensitivity detection ofcontaminants in corrosive gas systems in commercial settings.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method is provided fordetecting the presence of gaseous species in a gas sample containingcorrosive gas. The method comprises:

(a) selecting a spectral region wherein (i) the gaseous species has atleast one absorption feature and (ii) the corrosive gas has essentiallyno absorption features;

(b) providing a laser comprising a laser cavity and a gain medium whichresides therein, the gain medium outputting light having a wavelengthdistribution at least a portion of which is in the selected spectralregion;

(c) providing a gas sample cell having windows which are transparent tolight in the selected spectral region such that a beam of light can passthrough the gas sample cell;

(d) inserting the gas sample cell in the laser such that output lightfrom the gain medium passes through the gas sample prior to exiting thelaser cavity;

(e) inserting the gas sample containing the corrosive gas in the gassample cell such that output light from the gain medium passes throughthe gas sample, the gas sample sealed within the gas sample cell suchthat the corrosive gas does not react with the laser; and

(f) directing the output beam of the gain medium after exiting the lasercavity to a detector assembly for determining the presence and/orconcentration of the gaseous species in the gas sample.

The ILS gas detection system of the present invention preferablycomprises a pumping laser used to provide the optical excitementrequired to operate the ILS laser, a multimode ILS laser operated overthe wavelength region in which the species of interest absorbs, a gassample cell placed within the optical resonator cavity of the ILS laserto contain the corrosive gas, a wavelength dispersive spectrometercapable of spectrally resolving the output of the ILS laser, a detectorcapable of measuring the wavelength-resolved intensity of the ILS laseroutput, and an electronic circuit which can read the signal from thedetector and convert it into electronic signal that can be processed bya computer. The ILS gas detection system may also include a chopping,pulsing, or modulating device designed to periodically interrupt theintensity of the pumping laser beam and the output from the ILS laser.

In accordance with various aspects of the present invention,contaminants are detected optically at concentrations below 1part-per-million (ppm) and extending to a level below 1 part-per-billion(ppb) by using ILS techniques. For detecting water vapor in HCl, asolid-state laser with an ion-doped crystal medium and operating in the1300 nm to 1500 nm spectral region preferably serves as the detector. Agas sample containing gaseous contaminant species (water vapor) andcorrosive gas (HCl) is placed inside the optical resonator cavity of thelaser (between reflective surfaces or mirrors) and on one side of theactive medium. Laser media having Cr⁴⁺ :YAG and Cr⁴⁺ :LuAG are describedhere, but other gain mediums such as other ion-doped crystals havingmultiple longitudinal and transverse cavity modes can be used as well.Alternative lasers systems may be employed as well. For example, a diodelaser pumped solid state laser with an ion-doped crystal medium may beoptically configured to provide ILS detection.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the present invention will behereinafter described in conjunction with the appended drawing figures,wherein like designations denote like elements. The drawings referred toin this description should be understood as not being drawn to scaleexcept if specifically noted.

FIGS. 1A and 1B are schematic block diagrams of a contaminant detectorsystem in accordance with the present invention; FIG. 1A shows the basicconfiguration, while FIG. 1B shows that configuration as embodied in anexemplary embodiment shown in FIG. 2;

FIG. 2 is a more detailed schematic perspective view of the exemplaryembodiment of a contaminant detector system in accordance with thepresent invention;

FIGS. 3A-3C include schematic representations of simple laser devices;

FIGS. 3D-3F are graphs that represent the graphical spectral outputs(intensity versus wavelength) obtainable from the devices depicted inFIGS. 3A-3C, respectively;

FIG. 4 is a schematic perspective view of an ILS chamber including thechamber components depicted in FIG. 2, some of the components shown inpartially broken away fashion;

FIG. 5 is a perspective view of an exemplary ILS laser crystal holderand heat sink useful in connection with the contaminant detector systemshown in FIG. 2;

FIG. 6 is an enlarged perspective view of an exemplary embodiment of abeam shaping assembly including a chopper element which may beadvantageously used in connection with the contaminant detector systemshown in FIG. 2;

FIG. 7 depicts plots showing exemplary water absorption spectra in N₂and HCl gases over the wavelengths of 1433-1440 nanometers; and

FIG. 8 depicts plots showing ILS water absorption in N₂ and HCl gasesover the wavelengths of 1420-1434 nanometers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is now made in detail to a specific embodiment of the presentinvention, which illustrates the best mode presently contemplated by theinventor for practicing the invention. Alternative embodiments are alsobriefly described as applicable.

As previously briefly noted, the subject matter of the present inventionis particularly well suited for use in connection with the manufactureof semiconductor components, and thus, a preferred exemplary embodimentof the present invention will be described in that context. It should berecognized, however, that such description is not intended as alimitation on the use or applicability of the subject invention, butrather is set forth to merely fully describe a preferred exemplaryembodiment thereof.

In this regard, the present invention is particularly suited fordetection of contaminants. Contaminants as used herein refers tomolecular, atomic, radical, and ionic species such as may be present ingaseous materials, such as in the gaseous materials which are used inthe fabrication of silicon films, i.e. , inlet lines. Alternatively, theterm contaminant may also refer to the gaseous material itself, such as,for example when the detector of the present invention is used todetermine if a line (e.g., HCl line) has been sufficiently purged of thegaseous material.

In accordance with a preferred embodiment of the present invention andwith momentary reference to FIG. 1A, a gas (contaminant) detectionsystem 10 suitably comprises a pumping laser system A, an ILS laser andassociated chamber B, a spectrometer C, and a detector with associatedelectronics (e.g., computer, digital electronics, etc.) D. Moreparticularly, and with reference to FIGS. 1B and 2, pumping laser systemA suitably comprises a pumping laser 100, a beam shaping optics assembly200 and a beam modulation assembly 300; laser and chamber B suitablycomprises a chamber assembly 400 and an ILS laser 500; spectrometer Csuitably comprises a spectrometer assembly 600; and, detector D suitablycomprises a detector assembly 700 and a computer system 800. As will bedescribed more fully herein, gas detection system 10 advantageouslydetects gaseous species (contaminants) which are suitably contained in agas sample. In general, pumping laser driver system A pumps ILS laser500, preferably at or near the threshold level such that a laser beampasses through the gas sample thereby enabling the spectrum of the gassample to be obtained. This spectrum is suitably detected through use ofdetector/computer system D which, upon manipulation, enables thereliable and accurate determination of the presence and concentration athigh sensitivity levels of gaseous species (contaminants) which may becontained within the gas sample.

With reference to FIGS. 3A-3C, and in order to more fully explain thescientific principles utilized in accordance with a preferred embodimentof the present invention, the general principles of intracavity laserspectroscopy (ILS) are illustratively shown. As is known, in itssimplest terms, a laser can be described as containing a gain medium, inwhich optical gain is produced, and a resonator, comprised of opticalelements such as mirrors. Optical losses may appear in both the gainmedium and the optical elements comprising the laser cavity (e.g., theresonator). With particular reference to FIG. 3A, a laser device in itssimplest form can be schematically illustrated as including a gainmedium 1A around which respective mirrors 2A and 3A are placed. Mirrors2A and 3A are typically coated to have high reflectivity surfaces over abroad spectral range. For example, the mirror coating on mirror 2A maybe totally reflective, while the mirror coating on mirror 3A may bepartially reflective thereby permitting some light to escape from thelaser cavity. The spatial region between the reflective surfaces ofmirrors 2A and 3A in which the gain medium 1A is placed defines thelaser resonator or cavity, and in the context of the present inventionrelates to the so-called "intracavity region."

The intensity (I) of the laser output may be determined both by thewavelength region over which the gain medium 1A operates (λ) and thereflectivity of the resonator elements (e.g., mirrors 2A and 3A).Normally this output is broad and without sharp, distinctive spectralfeatures, as is shown in the plot of I versus wavelength (λ) provided inFIG. 3D, which corresponds to the laser of FIG. 3A.

By selecting different optical elements to form the laser cavity, thespectral output of the laser can be altered or "tuned." For example, andwith particular reference to FIG. 3B, a tuned resonator cavity mayinclude a diffraction grating 2B which replaces the highly reflectivemirror 2A shown in FIG. 3A. As shown, the laser device thereforeincludes diffraction grating 2B, mirror 3B, and a gain medium 1Bpositioned therebetween. In general, the result in spectral output fromthis tuned laser will be narrowed and appear as wavelengths within theoriginal spectral output of the laser defined by the gained medium 1Aand the mirrors 2A and 3A (FIG. 10A). For example, a schematic plot ofintensity (I) versus wavelength (λ) illustrating a narrowed output isdepicted in FIG. 3E.

The laser output can also be altered by placing gaseous molecules,atoms, radicals, and/or ions in either their ground or excited statesinside the optical resonator (e.g., cavity). With reference to FIG. 3C,a laser so configured may include a highly reflective minor 2C, apartially reflective mirror 3C with a gain medium 1C, and an intracavityabsorber 4 placed therebetween. In this case, intracavity absorber 4 maycomprise such gaseous species (e.g., the sample containingcontaminants). The effect of the intracavity gaseous species on thelaser output can be observed. For example, a plot of I versus λ for sucha device is shown in FIG. 3F. FIG. 3F comprises an absorption spectrumof the gaseous species contained within intracavity absorber 4. Thedistinct absorption features illustrated in FIG. 3F arise from theintracavity species losses against which the laser gain must compete.

Thus, the absorption spectrum of the intracavity species may appear inthe spectral output of the laser. In particular, the laser outputintensity (I) at wavelengths where the stronger intracavity absorptionfeatures compete effectively against the gain properties of theresonator are more reduced. As a result, as illustrated, instead of arelatively smooth continuous output, such as shown in FIG. 3D, astructured laser output such as shown in FIG. 3F may be observed. Thedecreases in intensity (I), as shown in FIG. 3F, are due to absorptionby the gaseous intracavity species, i.e., the more intense theabsorption features, the larger the decrease in the laser outputintensity. In accordance with the present invention, the absorptionspectrum obtained by intracavity laser measurements in which anintracavity absorber is employed can be utilized for the highsensitivity detection of such gaseous species. It has been found thateach gaseous species can be uniquely identified by its respectiveabsorption spectrum (signature) and thus can be used to confidentlyidentify such gaseous species (contaminant).

The present inventor has found that the appearance of the absorbingspecies (gaseous elements) within the laser resonator before and/orduring the competition between gain and losses which naturally occur asthe laser system approaches threshold give rise to enhanced detectionsensitivity through use of ILS. In view of the fact that the lossesassociated with the intracavity absorber become part of the competitionbetween the gain and losses within the laser, even a small absorbanceassociated either with a weak absorption transition and/or an extremelysmall absorber concentration is amplified dramatically during thegain/loss competition. As a result, such competition dearly appears inthe output of the ILS signal (see FIG. 3F). Thus, using theseprinciples, ILS can be utilized to detect both weak absorption and/orextremely small absorber concentrations.

ILS detection differs significantly from other spectroscopy methodswhich employ lasers. As described above, the output of a laser used forspectroscopy typically excites in a gaseous species, a secondaryphenomena which is then monitored. Alternatively, output of a laser maybe passed through a gaseous species and the absorption of selectedwavelengths in the output of the laser provides means for characterizingthe gas. In either case, the operation of the laser is separate from andunaffected by the gaseous species being measured.

With ILS detection, however, the operation of the laser is directlyaffected by the gaseous species. In this manner, the ILS laser 500itself acts as a detector. In particular, the output from the ILS laser500 as it exits the laser cavity contains spectroscopic informationabout the gaseous species. This mode of operation is unique to ILSdetection and the ILS laser 500.

Accordingly, ILS lasers 500 are distinctly different from conventionallasers and possess operational characteristics which are not typical ofconventional lasers. For example, absorbing species which produce lossare intentionally introduced into the laser cavity of ILS lasers 500.These absorbing species affect the operation of the ILS laser 500 andalter its output.

Also, unlike lasers employed in conventional applications, ILS lasers500 operate above but close to threshold (e.g., within 10% of thresholdpower). However, operating near threshold often causes the output of theILS laser 500 to be unstable. Accordingly, additional techniquesdirected to stabilizing the output of the ILS laser 500 may be required.

In contrast, conventional lasers typically operate well above thresholdto maximize output. Maximizing output, however, is not the objective ofILS lasers 500. Consequently, laser media which are inefficient and/ordo not produce high output power may be employed for ILS detection whensuch laser media are unfavorable for most other laser applications. Thepurpose of the ILS laser 500 is not to produce light, but to monitorloss within the laser cavity. As described above, mode competitioninside the laser cavity enables such loss within the ILS laser 500 to bedetected with enhanced sensitivity.

Since ILS detection possesses increased sensitivity beyond conventionaloptical spectroscopy techniques, interferences from background gaseshaving both weak absorption and/or extremely small absorberconcentrations may be significant, even if such interferences arenegligible with conventional spectroscopy techniques.

The detection of gases via ILS can be achieved by using a variety oflaser systems. (As used herein, the laser system includes both the ILSlaser 500 and the pump laser 100). These laser systems each shareseveral common properties which are required for extremely highdetection sensitivity. Prior art has identified three such properties.First, the laser systems exhibit multimode operation near the energythreshold for lasing. Second, the laser systems possess an operationalwavelength bandwidth that is substantially broad relative to theabsorption features of the gaseous species or contaminants (i.e.,molecules, atoms, radicals, and/or ions) being monitored. Third, thelaser systems maintain stable intensity and wavelength.

It will be appreciated that a variety of ILS laser systems havingdifferent physical and optical characteristics meet these above-listedcriteria for extremely high detection sensitivity. The differentphysical and optical characteristics of the laser systems may alsoprovide distinct advantages such as with regard to the experimentalconditions (e.g., data acquisition times) under which ILS measurementsare made. Additionally, these different physical and opticalcharacteristics may influence one or more of the following: (1) thegaseous species or contaminant (i.e., molecules, atoms, radicals, and/orions) that can be detected; (2) the respective concentrations of eachgaseous species that can be determined; and (3) the practical types ofsamples to which detection can be applied. Examples of the latterinclude the total pressure of the sample, the sample size, and theenvironment within which the sample is contained.

Against the backdrop of these general principles, in the context of thepresent invention, the present inventor has devised a commerciallyviable contaminant sensor system 10 which provides enhanced detection ofcontaminants in gaseous samples which contain corrosive gases (i.e.,gases which react with the components comprising the ILS laser 500).

With reference now to FIGS. 1A and 2, and in accordance with a preferredexemplary embodiment of the present invention, a gas detection system 10suitably includes laser driver 100 and an ILS chamber assembly 400 inwhich ILS laser 500 is contained. Spectrometer assembly 600 and adetector/computer system 700, 800 are suitably optically connected tothe output from ILS laser 500 whereat the absorption spectrum issuitably manipulated thus enabling the high sensitivity detection of thepresence and/or concentration of gaseous species (contaminants).

The ILS laser 500 operates in a wavelength region or spectral regionsuitable for detection of the contaminant contained within the gassample (e.g., water vapor). Within this spectral region a signatureabsorption spectrum can be obtained. Since gas phase spectroscopy hasbeen studied extensively, a large and directly useful literature existswhich shows how gaseous species can be uniquely identified.

It will be appreciated that ILS detection can be used to monitor onegaseous species (e.g., water vapor) in the presence of corrosive gas(e.g., hydrogen chloride) by selecting the appropriate spectral windowwithin which to measure absorption. In particular, a spectral regionmust be determined wherein (i) the contaminant or gaseous species has atleast one absorption feature and (ii) the corrosive gas has essentiallyno interfering absorption features. By interfering absorption featuresis meant absorption features which overlap the absorption feature usedto identify the gaseous species thereby causing the detectionselectivity between the gaseous species and the corrosive gas to becompromised. Many such spectral windows (where the corrosive gas has noabsorption or very weak absorption) exist within the absorption regionsof a gaseous species. Since absorption by the corrosive gas is extremelyweak (if measurable) within these spectral windows, ILS sensors 100operating at these wavelengths can be used to detect the gaseous specieswithout interference. Finding a wavelength region where such a spectralwindow exists, in order to avoid interfering spectroscopic signal, mayrequire significant efforts. In particular, the enhanced detectionsensitivity of ILS detection for measuring absorption in gases meansthat even interferences from background gases having weak absorptionand/or extremely small absorber concentrations may be significant; evenif such interferences are negligible with conventional spectroscopytechniques. Additionally, operation of an ILS laser 500 in a specificspectral region where a particular spectral signature exist may alsorequire considerable effort.

Nevertheless, unique spectroscopic identification of a specific gasspecies can be routinely expected. Thus, selectivity in the gaseousspecies detected can be achieved. The existence of these spectralwindows, however, may be evident only at high spectral resolution.Consequently, sufficient spectral resolution may be required to separateabsorption features that appear near each other.

It will be further appreciated that the wavelength position and width ofa specific spectral window depends directly on the specific gaseousspecies and the specific corrosive gas. For example, for detection ofwater vapor in hydrogen chloride (HCl), a spectral region may beselected from the wavelength region between about 1420 and 1434nanometers (nm). Alternatively, a spectral region may be selected fromthe wavelength region between about 1433 and 1440 nanometers.

Accordingly, the ILS laser 500 must output light having a wavelengthdistribution at least a portion of which is in the selected spectralregion.

In order to drive ILS laser 500, gas detection system 10 requires apumping source which delivers radiation of sufficient power and within asuitable wavelength region so as to optically excite the ILS laser at orslightly above its threshold. In this regard, it is important that ILSlaser 500 operate such that the gain in the laser medium exceeds theoverall optical losses, including those associated with the gain medium,mirrors, and non-mirror intracavity optical elements, as well as theabsorption of any gaseous species within the optical resonator cavity.Moreover, preferably laser 500 operates with multiple longitudinalmodes, i.e., over a broad wavelength region. Typically, a desirablebandwidth over which laser action occurs is between about 2 nm and about15 nm. While ILS laser 500 can also operate with more than onetransverse resonator mode, such is not necessary. In accordance with apreferred exemplary embodiment of the present invention, the laserdriver comprises an optical pumping laser 100. Suitably, the opticalparameters (e.g., average power density, peak power density, divergenceand beam diameter) of pumping laser 100 advantageously match the opticalrequirements of ILS laser 500. As will be appreciated, to do so it isnecessary to determine how many photons can be delivered within aspecific volume and at a given distance from the pumping laser 100 overa particular period of time. In general, in accordance with the presentinvention, such determinations are made in accordance with knowntheoretical and quantitative equations such that the pumping laser 100is suitably selected to advantageously match the optical characteristicsof ILS laser 500.

While other drivers may be utilized in the context of the presentinvention, preferably, a pumping laser 100 is selected on the basis ofits operational wavelength and on its optical parameters in a mannersuch that it can alone be used to excite ILS laser 500. As will bedescribed in greater detail hereinbelow, in cases where pumping laser100 is not effective alone to drive (e.g., pump) ILS laser 500, beammodification optics, such as beam shaping assembly 200, can be utilized.Examples of beam modification optics include diffractive optics,refractive optics, gradient index optics wherein the refractive indexvaries axially, gradient index optics wherein the refractive indexvaries radially, micro-optics, and combinations thereof. However, inthose cases where the radiation emanating from laser 100 suitablymatches the pumping requirements (e.g., mode volume) of the ILS gainmedium contained within ILS laser 500, such beam modification optics areunnecessary.

In accordance with one exemplary embodiment of the present invention,pumping laser 100 comprises a laser operating at a wavelength, λ_(p),(which is this case is a solid state crystal laser, i.e., a Nd:YAG,operating at approximately 1064 nm having an output power greater thanabout 2.8 watts with a TEM₀₀ transverse mode structure). Suitably, abeam E propagating from pumping laser 100 has a linear polarization andis rotatable perpendicular to the plane of propagation. Preferably, thedivergence of the output beam (beam E) from pumping laser 100 is on theorder of less than 0.5 mrad and evidences a beam diameter on the orderof less than 5 mm. A particularly preferred pumping laser 100 is modelT20-1054C from Spectra Physics of Mountain View, Calif. As will beexplained more fully hereinbelow, use of such a pump laser 100 typicallyrequires use of beam shaping optics assembly 200.

It should be appreciated that driver 100 may comprise any suitableoptical pumping source, either coherent or incoherent, continuous orpulsed, that will suitably excite ILS laser 500. As a result, even inaccordance with the previously recited preferred embodiment, pumpingsource 100 operates in a conventional manner and emits radiation over adesired frequency band and having a desired bandwidth.

For example, pumping laser 100 may comprise a diode laser, see e.g.,above-mentioned patent application Ser. No. 08/675,605 by G. H. Atkinsonet al, entitled "Diode Laser-pumped Laser System for Ultra-sensitive GasDetection Via Intracavity Laser Spectroscopy (ILS)". Use of a diodelaser as a pump laser 100, however, typically requires use of a beamshaping optics assembly 200.

With continued reference to FIG. 2, ILS laser 500, in the simplest casecomprises an optical resonator cavity defined by the entire optical pathlength between respective mirrors 501, 503, 505.

For gas samples containing corrosive gases, i.e., gases which chemicallyreact with one or more of the laser components, it is desirable toseparate the gas sample region from such components (e.g., gain medium,mirrors, etc.). In accordance with a preferred embodiment of the presentinvention, a separate sample system 400A may be advantageously utilizedto isolate the sample from the laser components.

With reference to FIGS. 2 and 4, in accordance with this preferredaspect of the present invention, sample system 400A preferably comprisesa gas sample cell body 406 suitably maintained within a gas sample cellholder 407. Respective cell windows 404 and 405 are suitably mounted onthe distal ends of gas sample cell body 406 and provide optical accessto the sample within the cell body. Windows 404 and 405 also suitablyseal cell body 406. As will be discussed in greater detail below, theregion in which system 400A is suitably placed is astigmaticallycompensated. Given this astigmatic compensation, windows 404 and 405 arenot "active" optical elements which significantly alter or perturb theoutput the ILS laser beam except with respect to transmission. An inletconduit 408 and an outlet conduit 409 are operatively connected to gascell body 406.

With reference to FIG. 4, couplings 408 and 409 are advantageouslyemployed to ensure efficient and effective passage of a gas sample intoand out of gas (contaminant) sample cell system 400A. Accordingly, thegas detection system 10 of the present invention can continuouslymonitor a flowing gas at variable pressures including high pressure. Inparticular, the use of the gas sample cell body 406 advantageouslyenables the operation of the ILS laser 500 when measuring gases having apressure which is different (i.e., higher or lower) than atmosphericpressure or the pressure for which the ILS laser was designed to lase.Without such a gas sample cell body 406, lasing would be difficult toachieve when monitoring a gas sample having a different pressure fromthe pressure at which the ILS laser 500 was aligned. Thus, the gassample cell body 406 allows stable operation of the ILS laser 500 for agas sample having a pressure in excess of atmospheric pressure or thepressure which the ILS laser was design to lase. Alternatively, the gassample may have a pressure less than atmospheric pressure or thepressure which the ILS laser 500 was design to lase (e.g., when a vacuumexists in the gas sample cell body 406). Additionally, the gas samplecell body 406 enables stable operation of the ILS laser 500 for a gassample having a pressure which fluctuates.

Suitably, cell body 406 comprises a stainless steel body havingdimensions suitably in the range of 10 to 90 millimeters (mm). Inaccordance with a particularly preferred aspect of the presentinvention, sample system 400A defines an opening (i.e., the opening inbody 406) having a diameter on the order of about one-eighth of an inch.Preferably, the opening in body 406 is symmetrically in the center ofgas sample cell body 406. Preferably, the diameter of the opening incell body 406 is suitably selected to be significantly larger than thediameter of the incoming beam such that optical alignment of gas samplesystem 400A may be easily obtained.

The thickness of windows 404, 405 is suitably selected to avoidinterferometric effects which may interfere with the quality of the ILSabsorption spectrum obtained through operation of the gas detectionsystem 10. In accordance with this aspect, the material used in formingwindows 404, 405 is optimally chosen to minimize absorption losses inthe region over which ILS laser 500 operates, such as in the range of1350 to 1550 nanometers (nm). For example, windows 404, 405 may beformed from an optically compatible material, such as Infrasil™, that ishighly polished. Windows 404, 405 are suitably oriented at Brewster'sangle so as to further minimize reflective losses from the windowsurfaces.

As so configured, gas sample cell 406 suitably permits beam H to passthrough the gaseous sample to be analyzed. Couplers 408, 409 withattached tubing are suitably selected to provide easy adjustment such asmay be required to realign and/or align windows 404, 405 within ILSlaser 500 without significantly altering the threshold pumpingconditions. The resonator cavity, in the case where system 400A isemployed, is suitably defined by the physical length between mirrors501, 503, 505 (including the laser crystal 507 and including the regionbetween windows 404, 405 as well as windows 404 and 405 themselves thatcomprise the sample system 400A).

It will be appreciated that it is necessary that any gases(contaminants) within chamber 400 that are to be detected are suitablyremoved or eliminated such that the absorption spectrum of the sampleobtained through use of the gas detection system 10 is accurate as tothe amount or presence of those gases (contaminants) within the gassample contained within system 400A. In accordance with a preferredaspect of the present invention, chamber 400 advantageously evidences asealed container which can be either purged of gas(es) (contaminant(s))to be detected, or evacuated to remove gas(es) (contaminant(s)) to bedetected, or in which the level of gas(es) (contaminant(s)) canotherwise be reduced below the level to be detected in the sample system400A. Continuous removal of the contaminants can be achieved forexample, by gettering, as described more fully below.

Referring to FIGS. 2 and 4, in accordance with a preferred embodiment ofthe present invention, ILS chamber 400 (excluding the sample system400A) suitably comprises a container base 401 and attachable top 410.Respective windows 402, 403 are suitably positioned in the walls of body401 in a suitable manner and position relative to the optical resonatorcavity defined therewithin. Container base 401 and top 410 suitablycomprise stainless steel or aluminum. Top 410 is advantageously securedto body 401 in accordance with any conventional technique suitable topermit evacuation, purging and/or further removal of contaminantstherewithin. For example, a gasket 410A or other suitable means togetherwith sealing devices (e.g., mechanical assists, metal seals, adhesives,and the like all not shown) may be employed for such purposes.Desirably, base 401 and top 410 are effectively sealed prior to deliveryto a user in a relatively tamper-proof manner.

For the purpose of purging or evacuating chamber 400, an inlet 411 forvacuum pumping and/or purging as well as an outlet 412 for vacuumpumping and/or purging are provided.

Windows 402, 403 are suitably disposed in the walls of container 401,thereby providing optical access to ILS chamber 400. Preferably, window402 is suitably provided with an antireflective (AR) coating on theorder of about 1000 to 1100 nm, and optimally about 1064 nm. On theother hand, window 403 preferably comprises an optical window without anAR coating. Window 402 is suitably designed to provide for maximumtransmission at the wavelength λ_(p), in this case, 1064 nm. Similarly,window 403 is suitably designed to provide maximum transmission over theoperational wavelength region of the ILS laser 500 (e.g., 1350 nm toabout 1550 nm).

More particularly, reducing gases (contaminants) in chamber 400(excluding sample system 400A) to an acceptable level may suitablycomprise purging or evacuating sealable container 401 with top 410 suchthat the level of gases (contaminants) is below that to be detected inthe gas sample within the sample system. It will be appreciated that theloss contributed by the gases in the chamber 400 will be comparable toloss contributed by the gases in the gas sample cell body 406 when theratio between (1) the concentration of gases in the chamber and (2) theconcentration of gases in the gas sample cell body is equal to the ratiobetween (1) the length of the cavity (i.e., between mirror 501 andmirror 505) and (2) the length that the ILS laser beam traverses in thecell body.

In such cases where the contaminant comprises water vapor, it isnecessary that water levels in chamber 400 be reduced below those whichare contained within the sample. In accordance with the presentinvention, detection levels of less than 1 part per billion (ppb) areobtainable in corrosive gas. While any now known or hereafter devisedmethod for removing contaminants (e.g., water) from chamber 400(excluding the sample system 400A) can be practiced within the contextof the present invention, preferably, chamber 400 is appropriatelysealed and inert gases, such as nitrogen are pumped therein. In someinstances, it may be necessary to further evacuate the chamber 400 so asto create a vacuum which removes substantially all contaminantscontained therein. Also, it may be useful to heat the chamber 400 whileevacuating. Application of such heating or "baking" will enable a higherlevel of vacuum to be achieved if the chamber 400 is subsequently cooledwhile continually being evacuated. In accordance with yet a furtheraspect of the present invention, a getter (not shown) may beadvantageously employed with chamber 400 to provide even furtherelimination of water within the chamber. As will be appreciated by thoseskilled in the art, a getter (e.g., a molecular sponge) having thecapacity for continuously absorbing water may be utilized to reduce thelevel of water (contaminants) below the water concentration that is tobe detected in the gas sample cell 406 (e.g., less than 1 ppb).

The gas sample is suitably communicated to system 400A by connecting agas line to connectors 408, 409 and feeding the gas into sample system400A.

As briefly mentioned above, ILS sensor 500 suitably optically detectsgaseous species (contaminants, e.g., water vapor) contained in a gassample placed within chamber 400. In accordance with the presentinvention, ILS laser 500 suitably comprises a crystal (gain medium) 507mounted in a crystal holder 508. Crystal 507 is suitably mounted incrystal holder 508 such that crystal 507 also is optimally placed withreference to the incoming beam. As previously mentioned, the incomingbeam is suitably shaped either by operation of pump 100 or through useof beam shaping assembly 200 such that incoming beam F suitably matchesthe mode volume of the ILS gain medium (e.g., crystal 507).

In general, ILS laser 500 is suitably configured such that the laserbeam in the intracavity region is substantially parallel (i.e.,astigmatically compensated) in the region where the beam is directed tothe gas sample, e.g., as contained within system 400A. While a varietyof optical configurations may be employed for this purpose, these mirrorconfigurations have been found to be particularly advantageous. Such aconfiguration permits the accurate astigmatic compensation of the ILSlaser beam thus permitting simultaneous meeting of the opticalconditions necessary to pump ILS laser 500 at the lasing threshold andgeneration of a laser beam which is substantially parallel as it isdirected to the gas sample, such as contained within system 400A.

In accordance with this aspect of the present invention, respectivemirrors 501, 505 and a folding mirror 503 are suitably employed for thispurpose. Mirror 501 preferably comprises an optical mirror having an ARcoating optimally centered about λ_(p), e.g., between about 1000 and1100 nm, optimally 1064 nm. Mirror 501 also has a coating thateffectively provides on the order of about 99.8% to about 100%reflectivity in the desired spectral region of operation of the ILSlaser 500 (e.g., about 1350 to about 1550 nm). Suitably, mirror 501comprises a concave mirror. For example, mirror 501 may have a radius ofcurvature (ROC) of about 10 centimeters (cm) and a diameter on the orderof about 1.0 to about 1.30 cm. Preferably, mirror 503 comprises afolding mirror which is configured similarly to mirror 501 and has asimilar reflection coating. In accordance with a preferred aspect of thepresent invention, mirror 503 has a coating suitable to achievereflectivity of about 99.8% to about 100% over the desired spectralregion (e.g., about 1350 to 1550 nm).

Preferably, mirror 505 comprises a flat mirror (ROC≈∞). With referenceto FIG. 2, one side of mirror 505, the side facing mirror 503 isadvantageously provided with a reflective coating in the desiredspectral region for lasing of the ILS laser 500, e.g., between 1350 andabout 1550 nm. The other side 505B of mirror 505 is suitably uncoated.

Preferably surfaces of mirror 505 are suitably wedged one against theother at an angle on the order of about 0.5 to about 3.0 degrees,optimally about 1.0 degree. The present inventors have found thatwedging such surfaces in this manner tend to minimize undesirablereflections which may lead to interference effects.

Mounts 502, 504, and 506 enable mechanical adjustment to optically alignthe ILS cavity within chamber 400. For example, the present inventorshave found that the efficiency of the ILS laser 500 is optimized for alaser configuration in which mirrors 501 and 505 are aligned for a beamincident angle of about 0° and mirror 503 is aligned for a beam incidentangle of about 12.5°.

Through the appropriate design, placement, and configuration of mirrors501, 503, and 505 beam H is substantially parallel (i.e., collimated) inthe region between mirrors 503 and 505. As a result, sample system 400Acan be inserted within the intracavity region without significantdeleterious effects in the performance of ILS laser 500. It will beappreciated, however, that the distance between any reflective surfaces(e.g., mirrors and windows) within the ILS laser 500 must not be suchthat any interference occurs inside the ILS laser. Interference patternsare produced if the distance between the reflective surfaces equals aninteger number of wavelengths comparable to the wavelength at which theILS laser crystal 507 operates.

As described above, ILS laser crystal 507 preferably operates in awavelength region suitable for detection of the contaminants containedwithin the gas sample (e.g., water vapor) over which a signatureabsorption spectrum can be obtained. As previously mentioned, lasercrystal 507 generally exhibits the properties of a multimode lasersystem. It will be appreciated that the mode spacing of output of thelaser crystal 507 is required to be small enough to accurately representthe absorption features of the gas sample. Light produced by lasercrystal 507 typically has a mode spacing of about 450 megahertz (MHz) toabout 550 MHz, preferably about 500 MHz, thus ensuring accurate spectralreplication of absorption bands. While any crystal may be utilized inthe context of the present invention, a Cr⁴⁺ :YAG or Cr⁺⁴ :lutetium(Lu)AG laser crystal may be optimally used in connection with thepresent invention for the detection of water vapor in HCl. Moreover,other hosts for the Cr⁺⁴ ions may be used and other doped ions intothese host crystals may be substituted. The present inventors have foundthat the low gain efficiency of a Cr⁴⁺ system in a garnet host (e.g.,YAG or LuAG) can be operated successfully as a laser using a crystalthat is on the order of about 10 to 30 mm in length and 5 mm indiameter. Preferably, the doped concentration is in the range of about0.10% to 0.30% in such garnet host. A particularly preferred laser gainmedium 507 comprises a Cr⁴⁺ :YAG or Cr⁴⁺ :LuAG crystal cut at Brewster'sangle to minimize reflective losses. More particularly, a Cr⁴⁺ :YAGcrystal cut at Brewster's angle (e.g., for a crystal refractive indexwhere n is about 1.82 and θ_(B) is about 61.20) having a crystal lengthof about 23 to 27 mm at about a 0.15% dopant level has been found to beparticularly advantageous in the context of gas detection system 10 inaccordance with the present invention. (Examples of other laser crystalsthat can be suitably employed in the present invention include aluminumoxide doped with titanium ions, Ti³⁺ :Al₂ O₃, barium lithium fluoridedoped with nickel ions, Ni²⁺ :BaLiF₃. A laser crystal comprising Ti³⁺:Al₂ O₃ outputs light having a wavelength between 0.680 to 1.100micrometers when pumped with light having a wavelength of 0.500micrometers. A laser crystal comprising Ni²⁺ :BaLiF₃ outputs lighthaving a wavelength between 1.3 to 1.6 micrometers when pumped withlight having a wavelength of 0.680 micrometers).

Laser crystals 507 currently available, while improving in efficiency,have considerable losses associated with them. The losses translate toheat. In accordance with the present invention crystal 507 suitably ismounted in a manner allowing for the effective removal of the heat thusgenerated in operation. It should be appreciated, however, that as theefficiency of laser crystals 507 continue to improve as new crystals aredeveloped, the need or requirements on heat removing devices will bereduced and likely, at some point, the losses will be small enough thatthe need to remove the heat may be eliminated all together. However,using crystals 507 presently available, ILS laser system 500 preferablyfurther comprises a heat sink system 500A.

With continued reference to FIG. 2 and additional reference to FIG. 5,heat sink system 500A is connected to mount 508 and crystal 507 (notshown in FIG. 5). As shown best in FIG. 5, holder 508 preferablycomprises a two-part holder suitably arranged to mechanically hold thelaser crystal 507. Heat sink system 500A preferably includes a copperheat sink bridge 510, a thermal electric cooler 509, and a thermalelectric sensor 511. Additionally, an electrical temperature controlinterface 512 is provided in the walls of the body 401 of the chamber400 (see FIG. 4). Mount 508 together with bridge 510, cooler 509, andsensor 511 serve to properly align crystal 507 with respect to the otheroptical elements comprising ILS laser 500, as well as enable control ofthe thermal properties of the crystal.

In accordance with the preferred aspect of the present invention, heatsink system 500A is in direct physical contact with crystal 507. Heatproduced by normal operation of crystal 507 through optical excitationoccasioned by beam F is effectively conducted away from crystal 507thereby maintaining a relatively constant operating crystal temperature.Preferably, crystal holder 508 comprises copper/aluminum which isoperatively connected to cooler 509 and heat sink bridge 510. Suitably,heat sink 510 comprises a copper heat sink located in body 401 ofchamber 400 such that excess heat is conducted away from crystal 507.Sensor 511 measures the temperature of holder 508, cooler 509, bridge510, and crystal 507 such that optimum operating temperatures aremaintained. In accordance with this aspect of the present invention,thermal management of crystal 507 is obtained, thereby eliminating theneed for coolant liquids which may unnecessarily compromise andcomplicate the operation of the gas detection system 10.

ILS laser system 500 is suitably arranged such that the angle (φ) of thebeam exciting crystal 507 and the reflected beam from mirror 503 is onthe order of about 20° to 30°, more preferably from about 23° to 27°,and optimally about 25°. This beam (beam H) is directed to the samplesystem 400A.

Output beam G from ILS laser 500 after passing through sample system400A is directed to spectrometer assembly 600. Such direction can beobtained, such as shown in FIG. 2, through use of a folding mirror 601suitably mounted in a mirror mount 602. Mirror 601 preferably comprisesa plane mirror containing a coating for high reflectivity in the desiredspectral region of operation of the ILS laser 500 (e.g., 1350 to 1550nm).

It will be appreciated that the output from the ILS laser 500 canalternatively be transmitted via an optical fiber link to a remote sitefor spectral analysis. In particular, beam G can be coupled into anoptical fiber or an optical fiber bundle. The output of the ILS laser500, after having passed through the gaseous species, is thereby carriedto the spectrometer assembly 600 which is located at the remote site.Under the proper conditions, it has been demonstrated that such opticalfiber transmission does not distort the spectral data.

With continued reference to FIG. 2, spectrometer assembly 600 comprisesdispersive gratings designed to spectrally resolve a coherent beam, inparticular, the absorption spectrum of the contaminant in the sample tobe detected. Suitably, the spectral dispersion of the spectrometer 600is sufficiently large to clearly resolve the absorption features of suchcontaminant, thus enabling the identification of the "signature" of eachcontaminant and the quantitative determination of the concentration ofthe contaminant. While any now known or hereafter devised spectrometermay be utilized in accordance with the present invention, preferablyspectrometer 600 comprises two diffraction grating assemblies 600A and600B operating in conjunction with an optical beam expanding assembly600C and a focusing lens assembly 600D. Optical beam expanding assembly600C preferably comprises lenses 603 and 605 suitably mounted within thegas detection system 10 through use of mounts 604 and 606. Lens 603preferably comprises a negative lens and lens 605 preferably comprises acollimating lens; each preferably having an AR coating centered aboutthe absorption spectrum of the contaminant in the sample to be detected,e.g., between about 1000 nm and 1500 nm, optimally 1400 nm.

Diffraction grating assemblies 600A and 600B suitably compriserespective diffraction gratings 607 and 609 mounted on respectivediffraction grating mounts 608 and 610. As will be appreciated, mounts608, 610 permit tuning and adjustment of diffraction gratings 607, 609within spectrometer assembly 600.

Focusing lens assembly 600D preferably comprises a lens 611 containingan AR coating at the wavelength at which the ILS laser 500 operates,e.g., between about 1000 nm and 1500 nm, optimally 1400 nm. The lens 611focuses the output of the spectrometer 600 onto multichannel arraydetector 701.

The spectral region over which the ILS laser 500 operates is produced byspectrometer assembly 600 and is displaced spatially across a planewhere the multichannel array detector 701 is suitably fixed on a mount702. An electronic board 703 containing the control and timingelectronics required to operate and read information from themultichannel detector 701 is operatively connected thereto. As a result,the entire spectrally dispersed absorption spectrum of the particularcontaminant sought to be identified through use of the gas detectionsystem 10 can be obtained. The positions and relative intensities of thespecific absorption features of the contaminant can be utilized touniquely identify the detected gas (contaminant) as well asquantitatively determine the amount of the gas (contaminant) sodetected.

The detector 701 may comprise, for example, an InGaAs multichannel arraydetector with 256 pixels having 100 μm spacing. The light detected bymultichannel detector 701 is preferably transduced into electronicsignals at each detector element (pixel) with signals thereaftertransferred to an analog-to-digital (A/D) converter 801 through board703. Converter 801 is suitably connected through a BNC connector andshielded cable 704 such that the accurate transfer of information isensured. Once the data is so converted, it is sent to a computer 802which may be suitably programmed to convert the electronic signals intospectral information i.e., spectral signatures identifying a particulargas (contaminant) and concentration of gases (contaminants).

Alternatively, the output of the ILS laser 500 having passed through thegaseous species to be monitored can be directed to a spectrometerassembly 600 having at least one dispersive optical element (e.g.,diffraction gratings 607 and 609) therein which can be scanned withrespect to wavelength. The output of the spectrometer assembly 600 canthen be directed to a single channel detector. The spectral signature ofthe gaseous species in the laser cavity is obtained by scanning thedispersive optical element while the light transmitted through thespectrometer assembly 600 passes through an appropriate aperture (e.g.,slit) placed in front of the single channel detector. The intensity ofthe light transmitted through the spectrometer assembly 600, i.e., theoutput of the spectrometer assembly 600, is recorded as the dispersiveoptical element is scanned.

With reference now to FIG. 2, as previously mentioned, the gas detectionsystem 10 detects the spectrally resolved region over which the ILSlaser 500 operates once pumping laser 100 causes the ILS laser tooperate at or near its threshold level. In cases where the opticalcharacteristics of driver 100 suitably match those of ILS laser 500, noadditional modification of the output of driver 100 is necessary.However, in those cases where the volume of the pumping radiation fromdriver 100 that is transferred to the gain medium (i.e., crystal 507)does not suitably match the volume that must be optically excited withinthe gain medium of ILS laser 500, beam modification system 200 can beutilized to facilitate such volume matching. In its simplest form, beammodification system 200 preferably comprises an optical telescope usefulto optimize the radiation delivered to ILS laser 500 by focusing therequired photon density into the correct location and volume of the gainmedium of the ILS laser. Specifically, beam modification system 200 isused to alter the pumping radiation of driver 100 to meet therequirements of ILS laser 500.

In accordance with the present invention, beam modification system 200may comprises a beam expanding telescope. Specifically, in such caseswhere pumping laser 100 comprises the preferred Spectra Physics diodepump solid state crystal laser and ILS crystal 507 comprises a Cr⁴⁺ YAGor a Cr⁴⁺ :LuAG crystal, beam expansion of the pumping laser outputmaybe necessary.

As shown best in FIG. 6, system 200 suitably comprises a series oflenses and adjustable apertures. In particular, a frame 201 is providedwhich suitably includes respective upstanding walls into whichrespective variable aperture devices 202 and 205 are suitably provided.While any appropriate means of varying the aperture of the opening intowhich the output (e.g., beam E) of laser 100 enters or exits system 200may be employed, suitably, devices 202 and 205 comprise conventionalaperture varying devices. System 200 also preferably includes a focusinglens 203 and a collimating lens 204. Preferably, lens 203 comprises afocusing lens and lens 204 preferably comprises a collimating lens bothof which have an AR coating at the wavelength, λ_(p), e.g., at about1000 to 1100 nm, optimally about 1064 nm. As shown in FIG. 6, lenses 203and 205 are suitably attached to frame 201 to permit their respectivealignment within beam E.

With continued reference to FIG. 2, in some applications, it may benecessary that the incoming beam be appropriately focused into the lasergain medium (e.g., ion-doped crystal or glass) 507 within ILS laser 500.In accordance with the preferred aspect of the present invention, afocusing lens 206 may be advantageously mounted in a laser lens mount207 such that the focusing lens is suitably located within the path ofbeam F. In accordance with a particularly preferred aspect of thepresent invention, focusing lens 206 suitably comprises an opticalfocusing lens with an AR coating centered about a wavelength λ_(p),e.g., between about 1000 and 1100 nm and optimally 1064 nm coating.Focusing lens 206 may be suitably configured to evidence a plano-convexor convex-convex configuration.

As will be appreciated by those skilled in the art, the quality of thequantitative information obtainable through use of the gas detectionsystem 10 depends, at least in part, on stable operation of ILS laser500. In the context of the present invention, the stability of laser 500depends directly on how reproducibly ILS laser 500 reaches threshold.Desirably, pumping laser 100 suitably pumps ILS laser 500 continuouslynear threshold where its greatest sensitivity may be obtained. However,not all drivers are capable of reliably operating in a continuousfashion. In addition, operating continuously tends to requiresubstantial effort to maintain amplitude and wavelength stability of theILS laser 500 which may have an adverse impact on cost and therebyproduce an adverse impact on the commercial viability of the gasdetection system 10.

As an alternative to operating ILS laser 500 continuously, and inaccordance with a preferred embodiment of the present invention, the ILSlaser is operated in a "pulsed mode" or a "chopped mode". As usedherein, the terms "pulsed mode" and "chopped mode" refer to processesfor reproducibly exposing ILS laser 500 (i.e., ion-doped crystal 507) topumping radiation such that the ILS laser will be switched on and off.Chopping corresponds to causing the pump radiation to alternate betweenzero intensity and a fixed intensity value at a fixed frequency and overa fixed (often symmetric) duty cycle. In contrast, pulsing correspondsto causing the pump radiation to alternate between zero intensity and anon-zero intensity (which is not necessarily fixed) over a duty cyclewhich may be varied and which is typically asymmetric. (Alternatively,the pump radiation can be modulated such that the intensity of the pumpbeam does not reach zero intensity but fluctuates alternately between atleast two intensity levels which brings the ILS laser 500 alternatelyabove and below threshold).

Through operation in the chopped mode or the pulsed mode, stableoperation of ILS laser 500 consistent with the quantitative spectral andconcentration measurements may be obtained in a commercially viablemanner. Such intensity modulation (e.g., interruption) can be achievedutilizing, among other things, a mechanically operated chopper, anacousto-optic modulator, a shutter, and the like.

Alternatively, the output intensity of pump laser 100 may be modulatedinstead of secondarily chopping the output beam (beam E). In particular,if the pump laser 100 comprises a diode laser, the electrical powersupplied to the diode laser pump laser can be modulated to alternatelyobtain voltages just above and below that required to cause the ILSlaser 500 to lase. Consequently, the ILS laser 500 will be turned on andoff.

While any now known or hereafter devised manner of producing a choppedmode or a pulsed mode can be utilized in accordance with the presentinvention, advantageously such modes are obtained through use ofmodulation assembly 300.

In accordance with this aspect of the present invention, beam E isperiodically prevented from reaching ILS laser 500 by a rotatingmodulator 301 which periodically blocks and transmits the pumping laserbeam E. Specifically, in FIG. 6, modulator 301 comprises a mechanicalchopper. Mechanical chopper 301 is suitably placed so that beam E ismodulated before reaching ILS laser 500. While modulator 301 may beadvantageously placed before or after beam modification system 200, inaccordance with a preferred embodiment of the invention, the chopper issuitably placed at the focal point within beam modification system 200.Alternatively, the chopper 301 may be placed in front of the pump laser100. As best illustrated in FIG. 2, chopper 301 is suitably mounted inframe 201 of the beam modification assembly 200.

It should be appreciated that chopper 301 could be replaced by anydevice, e.g., mechanical or electro-optical, which periodically blocksor modulates the pumping laser beam. As previously mentioned, inaccordance with the present invention, the intensity of the pumpingradiation emanating from pump laser 100 must only fall below thatrequired to make ILS laser 500 reach threshold and therefore, is notrequired to reach a zero value. It will be further appreciated, however,that the total optical pumping energy (i.e., the integrated intensity)delivered by the pump laser 100 to the ILS laser 500, during each periodof modulation, must remain constant.

With either the pulsed mode or chopped mode, the output of ILS laser 500which contains the absorption information, may be periodically sampled.Referring again to FIG. 2, in accordance with a preferred embodiment ofthe present invention, modulation assembly 300 comprises modulator 301(provided with appropriate electronic circuitry) and modulator 304 (alsoprovided with appropriate electronic circuitry). Modulator 301advantageously modulates the intensity of output beam E from pump laser100 and lens 203, while modulation device 304 suitably modulates theoutput beam of ILS laser 500 that exits chamber 400, therebyperiodically sampling the output of the ILS laser. Suitably, modulator301 alternately blocks pumping beam E from reaching ILS laser 500 gainmedium (e.g., crystal 507), while modulator 304 alternately blocks ILSlaser beam exiting chamber 400 from reaching both spectrometer assembly600 and detector assembly 700.

Suitably, chopper 301 rotates between an open and closed position.Chopper 301 is suitably driven by a chopper driver 303 connected tochopper through use of a suitable electrical connector(s) 302. As driver303 causes chopper 301 to rotate to the open position, beam E reachesILS laser 500, thereby bringing ILS laser 500 above threshold for laseractivity. ILS laser 500 continues to operate until driver 303 causeschopper 301 to rotate to the closed position, whereupon chopper 301effectively blocks pumping beam E from reaching ILS laser 500.

ILS laser 500 output exiting chamber 400 is suitably directed tomodulator 304. In accordance with various aspects of the presentinvention, modulator 304 comprises an acousto-optic modulator. It shouldbe appreciated, however that other available devices, for example,another mechanically operated chopper or even a shutter may be suitablyemployed for this purpose. As discussed above, to extract quantitativeinformation from the ILS laser 500 exiting beam, modulator 304periodically samples the output of the ILS laser which contains theabsorption data of contaminants (e.g., gaseous species) contained in theparticular sample. (It will be appreciated that instead of employingmodulator 304, detector assembly 700 may be alternately switched on andoff to periodically sample the output of ILS laser 500 as will bediscussed more fully below).

While the specific form of modulation is variable, use of modulationenables generation of a reproducible, effective optical path lengthwithin ILS laser 500. Stated another way, by varying the generation time(t_(g)), i.e., the time period over which intracavity mode competitionwithin ILS laser 500 is permitted to occur, the effective absorptionpath length within the intracavity resonator can be controlled andselected to achieve optimum quantitative application of the ILS gasdetector 10.

Advantageously, modulation of the output of the pump laser 100 isadvantageously synchronized with modulation device 304 such thatquantitative information from ILS laser 500 can be extracted in atime-resolved manner. Pump radiation E is effectively delivered to ILSlaser 500 intermittently by passing pump beam E through chopper 301.Delivering radiation intermittently alternately brings ILS laser 500above threshold and below threshold. After the generation time t_(g),i.e., elapses as ILS laser 500 reaches its threshold, the ILS laseroutput is deflected by modulator 304 to the entrance of spectrometerassembly 600 and detector assembly 700 for detection. However, ILS laser500 output beam G is deflected to spectrometer assembly 600 and detectorassembly 700 for only a short time interval determined by thesynchronization of modulation devices 301 and 304. The synchronizationof modulators 301 and 304 ensures that radiation from ILS laser 500 issampled over a well-defined time interval (t_(g)).

Synchronization of modulators 301 and 304 may be achieved by severalconventional methods such as, for example, through electronic control bya digital circuit (not shown) operated by computer 802 operativelyconnected to gas detection system 10. Typically, synchronization ofmodulators 301 and 304 will be suitable to generate generation times(t_(g)) on the order of less than about 300 to 500 microseconds (μsec),more preferably on the order of less than about 10 to 100 μsec, andoptimally on the order of less than about 1 μsec. Such synchronizationresults in the modulation assembly 300 allowing the output of the pumplaser 100 to pass uninterrupted when modulator 304 is closed. The timeinterval between when the output of the pump laser 100 is notinterrupted by the modulation assembly 300 and when modulator 304 opensis determined by t_(g).

The generation time, t_(g), can be varied without the use of modulator304 by pulsing the output of the pump laser 100. As described above,pulsing corresponds to causing the pump radiation to alternate betweenzero intensity and a non-zero intensity value over (which is notnecessarily fixed) over a duty cycle which may be varied, therebybringing the ILS laser 500 alternately below and above threshold.Accordingly, the ILS laser 500 is turned off and on. The duration overwhich the ILS laser 500 lases may be varied by changing the duty cycleof the output of pump laser 100; in particular, the duration over whichthe pump laser pumps the ILS laser to threshold. Accordingly, thegeneration time (t_(g)), i.e., the time period over which intracavitymode competition within ILS laser 500 is permitted to occur, is varied.In this case, the detector assembly 700 remains continuously activatedand the output of the ILS laser beam exiting chamber 400 is allowed tocontinuously reach the spectrometer assembly 600 and the detectorassembly.

As described above, however, the total optical pumping energy orintegrated intensity delivered by the pump laser 100 to the ILS laser500 during each period of modulation must remain constant, even thoughthe duration over which the of the ILS laser outputs light is changed.To maintain a constant total optical pumping energy, the intensity levelof the pump beam is adjusted with each different period of modulationover which t_(g) is to be varied. Accordingly, both the intensity of thepump beam and duration over which the laser diode pump laser 100 pumpsthe ILS laser 500 to threshold, are a changed to provide differentgeneration times.

Pulsing the output of the pump laser 100 can be achieved by eternallycontrolling the transmission of the pump beam with a "pulser".Alternatively, if the pump laser 100 is a diode laser, the outputintensity of the diode laser pump laser may be modulated by varying theelectrical power supplied to the diode laser. (As described above, theelectrical power supplied to a diode laser pump laser 100 can bemodulated to alternately obtain voltages just above and below thatrequired to cause the ILS laser 500 to lase).

Accordingly, the gas detection system 10 of the present invention mayinclude any of the following configurations each of which enables thegeneration time to be varied:

(1) The output of the pump laser 100 may be chopped with an externalchopper (e.g., chopper 301) and the detector 700 may be continuouslyactivated with transmission of the output from the ILS laser 500 to thedetector being controlled by a pulser (e.g., modulator 304) to enableperiodically sampling;

(2) The output of the pump laser 100 may be chopped with an externalchopper (e.g., chopper 301) and the detector 700 may be pulsed on andoff to enable periodical sampling of the output from the ILS laser 500;

(3) The output of the pump laser 100 may be pulsed with an externalpulser and the detector 700 may be continuously activated with theduration of the interaction between the output of the ILS laser 500 andthe gaseous species being controlled by the duration of the pulses fromthe pump laser which cause the ILS laser to lase;

(4) In the case where the pump laser 100 is a diode laser, the output ofthe diode laser may be pulsed by varying the electrical power suppliedto the diode laser and the detector 700 may be continuously activatedwith the duration of the interaction between the output of the ILS laser500 and the gaseous species being controlled by the duration of thepulses from the diode laser which cause the ILS laser to lase;

(5) In the case where the pump laser 100 is a diode laser, the output ofthe diode laser may be chopped by varying the electrical power suppliedto the diode laser and the detector 700 may be continuously activatedwith the transmission of the output from the ILS laser 500 to thedetector being controlled by a pulser (e.g., modulator 304) to enableperiodical sampling; and

(6) In the case where the pump laser 100 is a diode laser, the output ofthe diode laser may be chopped by varying the electrical power suppliedto the diode laser and the detector 700 may be pulsed on and off toenable periodical sampling of the output from the ILS laser 500.

In the embodiment of the gas detection system 10 of the presentinvention described above, the ILS laser 500 has a laser cavity formedfrom three mirrors (i.e., mirrors 501, 503, and 505) wherein mirror 503is a folding mirror. As described above, this configuration of threemirrors is designed to provide an astigmatically compensated orsubstantially parallel beam in the region between mirrors 503 and 505.

Alternatively, the gas detection system 10 of the present invention maycomprise an ILS laser 500 having a simplified laser cavity. In such analternative embodiment of the present invention, the laser cavity is notdesigned to provide astigmatic compensation. Rather, the laser cavitymay be formed between two mirrors thereby having a substantially linearconfiguration which does not provide astigmatic compensation; see e.g.,patent application Ser. No. 08/675,531, filed on even date herewith byG. H. Atkinson et al, entitled "Linear Cavity System for Ultra-sensitiveGas Detection Via Intracavity Laser Spectroscopy (ILS)". However, thelinear cavity design employed by such an alternative embodiment of thepresent invention enables a gas detection system 10 to be constructedwhich is substantially smaller and simpler. Consequently, such anembodiment of the gas detection system 10 of the present invention isless expensive to construct as well as easier to operate than theembodiment described above. Additionally, this alternative embodiment ofthe present invention based on a linear laser cavity can be constructedto be more rugged or mechanically stable as is required by manypractical applications.

By "linear laser cavity" is meant a laser cavity that is equivalent to alaser cavity formed between only two mirrors. In its simplest form, alinear laser cavity comprises a laser cavity formed between a firstmirror and a second mirror. It will be appreciated that any number ofadditional mirrors which are planar may be included to steer (i.e.,alter the path) of a beam which travels from the first mirror to thesecond mirror. The inclusion of these additional mirrors, however, doesnot modify the shape of the beam within the laser cavity (provided thatthe distance between the first mirror and the second mirror is notchanged). Accordingly, the inclusion of additional planar mirrors in alaser cavity of a laser does not affect the operation of the laser butmerely alters the manner in which the laser is physically configured.Consequently, a laser cavity formed between a first mirror and a secondmirror, having additional planar mirrors therebetween, is equivalent toa laser cavity formed solely between the first mirror and the secondmirror; removing the additional planar mirrors alters neither the shapeof the beam nor the operation of the laser. The use of such additionalplanar mirrors, however, may be employed to fit a laser cavity into apackage having spatial constraints.

Thus, there has been disclosed an apparatus for detecting the presenceand concentration of contaminants in a gas sample containing a corrosivegas by utilizing gas detection system 10. In accordance with a preferredembodiment of the present invention, a method for high sensitivitydetection is also disclosed herein. The method for detecting thepresence of a gaseous species in a gas sample containing a corrosive gasrequires that a spectral region be selected wherein (i) the gaseousspecies has at least one absorption feature and (ii) the corrosive gashas essentially no interfering absorption features. (As described above,an interfering absorption feature corresponds to an absorption featurewhich overlaps the absorption feature used for identifying the gaseousspecies, such that, detection selectivity between the gaseous speciesand the corrosive gas is compromised). In accordance with the method ofthe present invention, a laser comprising a laser cavity and a gainmedium 507 which outputs light having a wavelength distribution at leasta portion of which is in the selected spectral region, is provided. TheILS laser 500 is contained in a sample chamber 400. A gas sample cell406 having windows 404 and 405 which are transparent to light in theselected spectral region, such that a beam of light can pass through thegas sample cell, is also provided. The gas sample cell 406 is insertedin the ILS laser 500 such that output light from the gain medium 507passes through the gas sample prior to exiting the laser cavity. Thegases (contaminants) in sample chamber 400 are reduced to an acceptablelevel. The sample of gas to be detected is placed in the gas sample cell406. The ILS laser 500 is pumped at or near threshold and the opticaloutput from ILS laser 500 is periodically sampled, preferably viamodulation assembly 300. The absorption spectrum of the gases(contaminants) within the sample is measured with spectrometer 600 anddetection assembly 700. The absorption spectrum is analyzed to identifythe gaseous species (contaminants) and determine its concentrationwithin the sample utilizing computer/software system 800.

More particularly, reducing gases (contaminants) in chamber 400(excluding the gas sample cell 406) to an acceptable level may suitablycomprise purging or evacuating sealable container 401 with top 410 suchthat the level of gases (contaminants) is below that to be detected inthe gas sample within the gas sample cell 406. As discussed previously,other mechanisms for reducing the level of gases (contaminants) may beutilized provided they can reduce the level to an acceptable level.Preferably, container base 401 is sealed to top 410 and contaminantscontained therein are effectively removed (or reduced to an acceptablelevel). Desirably, base 401 and top 410 are effectively sealed prior todelivery to a user in a relatively tamper-proof manner.

A sample is suitably communicated to gas sample cell 406 by connecting agas line to connectors 408, 409 and feeding the gas into the gas samplecell (for example, when the sample comprises a corrosive or reactivegas).

Pumping ILS laser 500 at or near threshold, more particularly, comprisesselecting the correct pump laser 100 power, focusing conditions at lasercrystal 507 utilizing beam modification optics 200 and lens 206, andmodulation conditions utilizing modulator system 300. The method fordetecting gaseous species in accordance with the present inventionfurther comprises driving ILS laser 500 at threshold or near to butabove threshold. In accordance with the present invention, driver 100suitably pumps ILS laser 500. Where necessary, pumping beam E issuitably shaped by beam shaping assembly 200 to meet the opticalrequirements of ILS laser 500. Further, where gas detection system 10 isoperated in a pulsed or chopped mode, as described above, modulationassembly, and in particular, modulator 301 may periodically interruptpump beam E thereby preventing beam E from reaching ILS laser 500. BeamF output from modulator 301 and beam shaping assembly 200 is suitablydirected to ILS laser 500.

In accordance with this method, as beam F enters chamber 400 throughwindow 402 disposed in the wall of sealed container body 401, beam F issuitably directed to ILS laser 500. Additional focusing and direction ofbeam F may suitably be achieved as beam F passes from window 402 tofocusing lens 206, where focusing lens 206 suitably focuses beam F anddirects it through mirror 501. Beam F suitably pumps crystal 507 at ornear threshold, and the output beam is suitably directed to the gassample within the gas sample cell 406, such as by mirrors 503 and 505.The exiting beam, containing the absorption data from the gas(contaminant) sample, then exits gas chamber 400 through window 403suitably disposed in a wall of sealed container body 401.

ILS laser 500 may be operated in a pulsed mode or a chopped mode usingmodulator 304, suitably synchronized to modulator 301, and whichperiodically samples the output beam from the ILS laser and passes thesampled output thus obtained to spectrometer assembly 600 and detectorassembly 700. Alternatively, in the case where the pump laser 100comprises a diode laser, the electrical power supplied to the diodelaser pump laser may be modulated and synchronized with modulator 304.Suitably, mirror 601 directs sampled output beam G from ILS laser 500 tospectrometer assembly 600 and detector assembly 700. Alternatively,instead of using modulator 304, detector assembly 700 may be switched onand off to sample the output from ILS laser 500.

The method for detecting gaseous species in accordance with the presentinvention further comprises analyzing the output beam G sampled from ILSlaser 500. Preferably, spectrometer assembly 600 spectrally resolves anddetector assembly 700 suitably analyzes beam G from sampled ILS laser500. Spectrometer assembly 600 suitably spectrally disperses beam G fromILS laser 500 through beam expanding assembly 600C, diffractionassemblies 600A, 600B and focusing assembly 600D. Spectrally-resolvedILS absorption data exiting spectrometer assembly 600 is suitablydisplaced spatially to be detected by detector assembly 700 comprisinge.g., multichannel detector 701.

It will be appreciated that the gas detection system 10 and method ofthe present invention can be utilized to obtain the absorption spectrafor contaminants, such as water vapor, in a corrosive or reactive gas,such as HCl, or a non-corrosive gas, such as N₂, over a variety ofwavelength regions. In particular, FIGS. 7 and 8 show that thesignatures of a water vapor in each environment (corrosive ornon-corrosive) can be obtained through operation of gas detection system10. Specifically, FIG. 7 shows a plot of normalized laserintensity/absorption vs. wavelength for water in HCl (curve 900) and inN₂ (curve 902) over the region of about 1433 to 1440 nm while FIG. 8shows a similar plot of normalized laser intensity/absorption vs.wavelength for water in HCl (curve 904) and in N₂ (curve 906) over thewavelength region of about 1420 to 1434 nm. The spectrum displayed inFIGS. 7 and 8 were obtained with the spectrometer assembly 600 and bymeasuring the output of diode 701. As will be appreciated, each datapoint illustrated results from a variety of measurements.

The data presented in the plots shown in FIGS. 7 and 8 demonstrate thatwater can be successfully detected in hydrogen chloride using the methodof the present invention. Additionally, the observation that the waterabsorption spectra are the same, regardless of whether the measurementis made in hydrogen chloride or nitrogen, demonstrates that there are nospectral interferences from hydrogen chloride even though it is presentat an extremely high concentration (e.g., one atmosphere totalpressure).

Given the relationship between intensity and concentration, once acharacteristic signature of the contaminant gas, e.g., water vapor, isobtained, the concentration of the contaminant contained within thesample can be readily obtained. In accordance with the presentinvention, computer 802 can be suitably programmed to interpret the dataand provide an output indicative of the presence and/or concentration ofthe contaminant contained within the sample.

(It will be appreciated that the absorption feature(s) found in thespectral signature must be calibrated. Since intracavity laserspectroscopy offers increased sensitivity beyond prior art methods, weaktransitions previously not measured may become measurable for the firsttime with the gas detection system 10 of the present invention. In suchcases, these weak transitions can be used to identify the spectralsignature and certify the presence of the gaseous species. Such weaktransitions can also be calibrated by the gas detection system 10thereby enabling the concentration of the gaseous species to bedetermined by the intensity of the absorption feature(s) correspondingto these weak transitions).

While the present invention is shown above as applied to obtaining theabsorption spectra for contaminants, such as water vapor, in a corrosivegas, such as HCl, it will be apparent by those skilled in the art thatother suitable contaminants and corrosive or reactive gases may also beemployed in the practice of the present invention. Examples of corrosivegases suitably employed with the present invention include, but are notlimited to, the following: N₂ O, NO, NO₂, HONO, HNO₂, SO, SO₃, H₂ SO₄,Cl₂, ClO, Cl₂ O₂, HOCl, PH₃, OCS, HI, HF, HBr, BCl₃, NF₃. BCl₂, BCl,SO₂, BF₃, Br₂, I₂, F₂, O₃, AsH₃, NH₃, SiH₄, B₂ H₄, HNO₃, HCN, HNC, H₂ S,COF₂, and CH_(4-x) X_(x), where X is F or Cl and x equals 1 to 4. It isnot intended that the corrosive or reactive gases specifically disclosedherein, including those listed above, are to be exhaustive; rather,other corrosive or reactive gases may be employed as is suited to theparticular use contemplated. For example, the corrosive gas may compriseother corrosive gases employed in the fabrication of semiconductorcomponents. Additionally, it will be appreciated that the corrosive gasmay comprise a combination of any number of corrosive gases such asthose listed above. It will further be appreciated that the gaseousspecies to be detected may comprise, but is not limited to, one or moreof the above listed corrosive gases.

Those skilled in the art will appreciate that the detection levelsavailable through practice of the present invention generally exceedthose which are obtainable through use of conventional devices.Moreover, gas detection system 10 can be used in-line and obtain ready,near real-time measurement of the presence and amount of the contaminantcontained in a specific corrosive sample, thus addressing the manydisadvantages associated with the use of such conventional devices. Inparticular, the method of the present invention provides rapid, in situwater vapor detection within gas samples containing corrosive gases atdetection levels which are not available in prior art.

It should be understood that the foregoing description relates topreferred exemplary embodiments of the invention, and that the inventionis not limited to the specific forms shown herein. Various modificationsmay be made in the design and arrangement of the elements set forthherein without departing from the scope of the invention as expressed inthe appended claims. Moreover, the application of gas detection system10 as well as the location of the ILS gas detector, e.g., in asemiconductor fabrication assembly, can vary as may be desired. Forexample, the specific placement of the various elements within the ILSchamber 400 and the gas detection system 10 itself may be modified solong as their configuration and placement suitably enables opticalexcitation of ILS laser 500 in a readily reproducible manner. These andother modifications in the design, arrangement, and application of thepresent invention as now known or hereafter devised by those skilled inthe art are contemplated by the amended claims.

What is claimed is:
 1. A method for detecting the presence of gaseousspecies in a gas sample containing corrosive gas, comprising the stepsof:(a) selecting a spectral region wherein (i) said gaseous species hasat least one absorption feature and (ii) said corrosive gas hasessentially no interfering absorption features; (b) providing a lasercomprising a laser cavity and a gain medium which resides therein, saidgain medium outputting light having a wavelength distribution at least aportion of which is in said selected spectral region; (c) providing agas sample cell having windows which are transparent to light in saidselected spectral region such that a beam of light can pass through saidgas sample cell; (d) inserting said gas sample cell in said laser suchthat light output from said gain medium passes through said gas samplecell prior to exiting said laser cavity; (e) inserting said gas samplecontaining said corrosive gas in said gas sample cell such that lightoutput from said gain medium passes through said gas sample, said gassample sealed within said gas sample cell such that said corrosive gasdoes not react with said laser; and (f) directing said light output fromsaid gain medium after exiting said laser cavity to a detector assemblyfor determining the presence and/or concentration of said gaseousspecies in said gas sample.
 2. The method of claim 1 wherein said gainmedium comprises an ion-doped crystal.
 3. The method of claim 2 whereinsaid gain medium is pumped by a pump laser selected from the groupconsisting of a solid state crystal laser and a diode laser.
 4. Themethod of claim 1 wherein said corrosive gas comprises a gas selectedfrom the group consisting of HCl, N₂ O, NO, NO₂, HONO, HNO₂, SO, SO₃, H₂SO₄, Cl₂, ClO, Cl₂ O₂, HOCl, PH₃, OCS, HI, HF, HBr, BCl₃, and CH_(4-x)Cl_(x), where x equals 1 or
 2. 5. The method of claim 4 wherein saidcorrosive gas comprises HCl.
 6. The method of claim 1 wherein saidgaseous species comprises water.
 7. The method of claim 5 wherein saidgaseous species comprises water and said selected spectral region isselected from the wavelength region between about 1420 and 1440nanometers.
 8. The method of claim 7 wherein said selected spectralregion is selected from the wavelength region between about 1420 and1434 nanometers.
 9. The method of claim 7 wherein said selected spectralregion is selected from the wavelength region between about 1433 and1440 nanometers.
 10. A gas detection system for detecting the presenceof gaseous species in a gas sample containing corrosive gas,comprising:(a) a laser cavity; (b) a gain medium which resides therein,said gain medium outputting light having a wavelength distribution atleast a portion of which is in a spectral region wherein:(i) saidgaseous species has at least one absorption feature, and (ii) saidcorrosive gas has essentially no interfering absorption features; (c) agas sample cell having windows which are transparent to light in saidspectral region such that a beam of light can pass through said gassample cell, said gas sample cell being contained in said laser suchthat light output from said gain medium passes through said gas samplecell prior to exiting said laser cavity; (d) means for inserting saidgas sample containing said corrosive gas in said gas sample cell suchthat said light output from said gain medium passes through said gassample prior to exiting said laser cavity; (e) means for sealing saidgas sample within said gas sample cell such that said corrosive gas doesnot react with said laser cavity or said gain medium; and (f) a detectorassembly for determining the presence and/or concentration of saidgaseous species in said gas sample, said light output from said laserbeing directed to said detector assembly.
 11. The gas detection systemof claim 10 wherein said gain medium comprises an ion-doped crystal. 12.The gas detection system of claim 11 wherein said gain medium is pumpedby a pump laser selected from the group consisting of a solid statecrystal laser and a diode laser.
 13. The gas detection system of claim10 wherein said laser cavity is contained within a chamber configured tobe evacuated of said gaseous species to be detected.
 14. The gasdetection system of claim 10 wherein said gas sample is contained in aregion of said laser cavity which is astigmatically compensated toreduce astigmatism in said light output from said gain medium.
 15. Thegas detection system of claim 10 wherein said corrosive gas comprisesgas selected from the group consisting of HCl, N₂ O, NO, NO₂, HONO,HNO₂, SO, SO₃, H₂ SO₄, Cl₂, ClO, Cl₂ O₂, HOCl, PH₃, OCS, HI, HF, HBr,BCl₃, and CH_(4-x) Cl_(x), where x equals 1 or
 2. 16. The gas detectionsystem of claim 15 wherein said corrosive gas comprises HCl.
 17. The gasdetection system of claim 10 wherein said gaseous species compriseswater.
 18. The gas detection system of claim 16 wherein said gaseousspecies comprises water and said spectral region is selected from thewavelength region consisting of the wavelength region between about 1420and 1440 nanometers.